1. Field of the Invention
The present invention relates to a sensing device detecting a substance to be sensed by using a piezoelectric vibrator.
2. Description of the Related Art
As a device to perform detection and measurement of a substance to be sensed in a sample solution, there is known a sensing device using a QCM (Quarts Crystal microbalance), an outline of an example of such a sensing device being illustrated in FIG. 15. A sensing device 100 of FIG. 15 has two crystal vibrators 101, 102, and those crystal vibrators 101, 102 are formed in a common crystal piece 103. It is configured that a substance to be sensed is adsorbed to a surface of the crystal vibrator 101 and that the substance to be sensed is not absorbed to a surface of the crystal vibrator 102. Those crystal vibrators 101, 102 individually oscillate by a first oscillation circuit 104 and a second oscillation circuit 105 each connected to the respective crystal vibrators 101, 102. Outputs of the respective crystal vibrators 101, 102 are time-divided by a switch 107 and taken into a measuring part 108 of frequency, and the measuring part 108 computes a difference value of frequencies of the crystal vibrators 101, 102.
As a result of having the common crystal piece 103, the crystal vibrators 101, 102 receive an influence of disturbance such as a vibration outside the device similarly to each other. Thus, the computed difference value of frequency reflects a frequency change due to absorption of the substance to be sensed, and detection and measurement of the substance to be sensed have been performed based on the difference value.
Incidentally, in a biochemistry examination of a hospital or an investigation by a reagent manufacturer, there is a demand for performing measurement of a substance to be sensed contained in a sample solution having a comparatively high viscosity. More specifically, an investigation is being carried on in which, for example, human saliva is taken and detection and measurement of markers of various diseases contained in the saliva are performed thereby to diagnose the disease of a patient. It is considered to use the above-described sensing device 100 for performing such an investigation.
It is found, however, that if the sensing device 100 performs measurement of a sample solution with a high viscosity, change amounts of frequency due to viscosities are different in the one crystal vibrator and in the other crystal vibrator, leading to an apprehension that a measurement accuracy is reduced as a result. This is because a difference in impedances of conductive paths from the oscillation circuits 104, 105 to electrodes formed in the crystal piece 103 emerges as a difference in a value of a crystal impedance (CI) to be a value of C0 or R1 at a time that the crystal vibrator is viewed in an equivalent circuit, and it is because even if a slight difference exists in a characteristic of each circuit between the oscillation circuits 104, 105, that difference is reflected.
Patent Document 1 describes a sensing sensor in which a plurality of vibrators are provided in one crystal piece, and lengths of conductive paths from oscillation circuits to respective vibrators are different from each other, and thus the above-described problem cannot be solved. Further, Patent Document 2 describes that one oscillation circuit is used for a plurality of vibrators and that a connection is temporally switched, but a means for solving the above-described problem is not described.    [Patent Document 1] Japanese Patent Application Laid-open No. 2000-338022    [Patent Document 2] Japanese Patent Application Laid-open No. 2009-31232